Perforin-mediated cytotoxicity is essential for clearance of primary LCMV infection. BALB/c-perforin-deficient (PKO) mice survived LCMV infection by deleting NP(118)-specific CD8(+) T cells whereas vaccination of PKO mice with Listeria expressing NP(118) generated a stable memory CD8(+) T cell population. However, >85% of vaccinated BALB/c-PKO mice died after LCMV infection. Mortality was associated with enormous expansion of NP(118)-specific CD8(+) T cells in both lymphoid and nonlymphoid tissues and aberrant CD8(+) T cell cytokine production. Depletion of CD8(+) T cells or treatment with anti-IFNgamma antibody rescued vaccinated mice from mortality. Thus, perforin was essential for resistance to secondary LCMV infection, and, in the absence of perforin, vaccination resulted in lethal disease mediated by dysregulated CD8(+) T cell expansion and cytokine production.